Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. While significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys, alternative materials have been proposed. For example, materials containing silicon, such as those with silicon carbide (SiC) as a matrix material or a reinforcing material, are currently being considered for high temperature applications, such as combustor and other hot section components of gas turbine engines. In many applications, a protective coating over the Si-containing material is beneficial, an example of which is a thermal-insulating layer to reduce the operating temperature and thermal gradient through the material. Additionally, such coatings can provide environmental protection by inhibiting the major mechanism for degradation of silicon carbide in a corrosive environment, namely, the formation of volatile silicon monoxide (SiO) and silicon hydroxide (Si(OH).sub.4) products. On this basis, besides low thermal conductivity, a critical requirement of a coating system for a SiC-containing material is low activity of silica (SiO.sub.2) in its composition. Other important properties for the coating material include a coefficient of thermal expansion (CTE) compatible with the SiC-containing material, low permeability for oxidants, and chemical compatibility with SiC and silica scale. Consequently, the coating essentially has a dual function, serving as a thermal barrier and simultaneously providing protection from the environment. For this reason, such a coating system will be referred to herein as a thermal/environmental barrier coating system, or TEBC.
While various coating systems have been investigated, each has exhibited shortcomings relating to the above-noted requirements and properties for compatibility with a Si-containing material. For example, an yttria-stabilized zirconia (YSZ) coating serving as a thermal barrier layer exhibits excellent environmental resistance by itself, since it does not contain silica in its composition. However, YSZ exhibits high permeability to oxygen and other oxidants. In addition, YSZ cannot be adhered directly to silicon carbide because of a coefficient of thermal expansion mismatch. As a result, mullite (3Al.sub.2 O.sub.3.multidot.2SiO.sub.2) has been proposed as a bond coat between SiC-containing substrate materials and ceramic coatings such as YSZ in order to compensate for differences in coefficients of thermal expansion.
Processibility by plasma spraying and good adhesion to silicon-based ceramics and ceramic composites have been shown for mullite. However, thermally-sprayed mullite bond coats have exhibited a large number of through-thickness cracks which serve as fast paths for the transport of corrosive species to the underlying substrate, and coating failures caused by extensive interfacial oxidation have been observed. The presence of the through-thickness cracks in mullite coatings was unexpected, because stoichiometric mullite (3Al.sub.2 O.sub.3.multidot.2SiO.sub.2) has a coefficient of thermal expansion fairly close to that of silicon-based ceramic composite materials. Accordingly, there is a need for the prevention of through-thickness cracks in mullite bond coats for TEBCs on silicon-based materials.